** This is designed to fit on a 4" receiver / adapter, not the 4" hose on a dust collector ** This adapter fits on a 4" dust collector receiver / adapter to adapt to a shop-vac 2.5" hose. The shop-vac end of the adapter is intended for the tapered fitting that is included in the shop-vac hose kit. There is an optional set screw hole in the fitting to allow securing the fitting to the 4" dust collector end.